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Solubility Product Principle. Dealing with the equilibrium of sparsely soluble solids. How do we deal with solids?. The solids are only slightly soluble (very little dissolves in water). What does dissolve behaves as a strong electrolyte (100% dissociation).

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solubility product principle

Solubility Product Principle

Dealing with the equilibrium of sparsely soluble solids

how do we deal with solids
How do we deal with solids?
  • The solids are only slightly soluble (very little dissolves in water).
  • What does dissolve behaves as a strong electrolyte (100% dissociation).
  • Solutions become saturated and solid may remain at bottom of container.
  • Can solubility be manipulated?
let s consider silver chloride
Let’s consider silver chloride

AgCl (s)  Ag+ + Cl-

  • What is the equilibrium constant?

Ksp = (Ag+) (Cl-)

  • Now what about the value of the Ksp?

Ksp = 1.6 x 10-10

  • Can we calculate the solubility, s?

s

s

s

= s2

s = (Ksp)1/2 = 1.3 x 10-5 M

write the dissociation of the solids and the k sp expressions
Write the dissociation of the solids and the Ksp expressions
  • AgBr (s) 
  • CaF2 (s)
  • Fe(OH)3 (s) 

Ag+ + Br-

Ksp = (Ag+)(Br-)

Ca++ + 2F-

Ksp = (Ca++)(F-)2

Fe+3 + 3OH-

Ksp = (Fe+3)(OH-)3

what is the ph of a bottle of milk of magnesia a common antacid
What is the pH of a bottle of Milk of Magnesia, a common antacid?

Milk of Magnesia is Mg(OH)2 with Ksp = 8.9 x 10-12

Mg(OH)2 (s)  Mg++ + 2OH-

s

s

2s

Ksp = (Mg++)(OH-)2 = s(2s)2 = 4s3

s = (Ksp/4)1/3 = 1.3 x 10-4 M

(OH-) = 2s = 2.6 10-4 M

pOH = 3.58 pH = 10.42

slide6

Would the solubility of AgI be the same or different in a solution of NaI? If different, how?

To address this you need to write a reaction and consider Le Chatelier’s principle-

AgI(s)  Ag+ + I-

The addition of the NaI causes the reaction to shift to the left. Hence the solubility decreases!

slide7

A beaker with NaI, where the aqueous I- is radioactive, and a crystal of AgI is placed in the beaker, where the I- in the solid is non-radioactive.

I- (aq)

Na+ (aq)

I- (s)

Ag+ (s)

The beaker is allowed to sit for a period of time.

This slide is meaning-less in black and white, you need to view the on-line version!

slide8

After a period of time, what do you notice about the distribution of the radioactive I-?

Explain the observation.

This slide is meaning-less in black and white, you need to view the on-line version!

what happened in the beaker
What happened in the beaker?

AgI (s)  Ag+ + I-

dissolves to saturate the aqueous NaI solution with AgI

However, after time passes, the radioactive I- is found in the solid AgI.

The process is dynamic, as AgI is constantly dissolving and crystallizing.

what influences the solubility
What influences the solubility?
  • What would happen if we added HCl to the AgCl solution?
  • What would happen if we added NH3 to the AgCl solution?

Decreases solubility

The addition of ammonia adds a new twist! As the Ag+ reacts to form a complex ion as shown below:

Ag+ + 2NH3 Ag(NH3)2+ K = ?

Free silver ion

Complexed silver ion

the formation of complex ions
The formation of complex ions

Ammonia will react with silver (I) ion:

Reactant or product favored?

Ag+ + NH3 Ag(NH3)+ K1 = 2.1 x 103

Ag(NH3)+ + NH3Ag(NH3)2+ K2 = 8.2 x 103

Ag+ + 2NH3Ag(NH3)2+ Kf = K1K2 = 1.7 x 107

Thiosulfate, used in photography, is an even stronger complexing agent. Why?

Ag+ + 2S2O3-2Ag(S2O3)2-3 Kf = 2.9 x 1013

Formation constant

slide12

Let’s examine some reactions:

Add these two reactions

AgCl (s)Ag+ + Cl- Ksp

Ag+ + 2NH3 Ag(NH3)2+ Kf

AgCl (s) + 2NH3  Ag(NH3)2++ Cl- KspKf

How is the solubility of AgCl (s) influenced?

According to Le Chatelier’s principle- the solubility of AgCl (s) increases as the (NH3) is increased.